46 GENETICS [Bot. Absts.. Vol. V, 



morphological structures of two single animals, reproduce by transverse fission. Favorable 

 environmental conditions necessary for continued existence of twins, i.e., do not survive in 

 competition with single animals. Selection produced striking increase in percentage of twins 

 in pedigreed culture from single twin animal. Division rate of twins similar to that of normal 

 animals. Miscible condition of twin cytoplasm handed on to twin progeny but is quickly 

 lost in single animals derived from twins, kept under identical environmental conditions. 

 Under favorable environmental conditions twin strains breed indefinitely. Pairing, canni- 

 balism, twin formation, occur among animals in similar physiological condition, these phenom- 

 ena therefore interpreted as abortive attempts to undergo syngamy, failure due to amicro- 

 nucleate condition. Inability to undergo syngamy has no effect on viability of race. [See 

 also next preceding Entry, 347.] — Austin R. Middleton. 



349. De Vries, H. Phylogenetische und gruppenweise Artbildung. [Phylogenetic and 

 group-wise species-formation.] Flora 11-12 (Festschr. E. Stahl) : 208-226. 1918.— Under the 

 term "gruppenweise Artbildung" de Vries understands the formation of a species within a 

 genus. There are also frequent transitions such as the reappearance of the same mutation 

 within a species. For example, the occasional appearance of a peloric form of Linaria 

 vulgaris. For the study of "group-wise" species formation the genus Oenothera offers excel- 

 lent material. The mutations observed in this genus can be divided into general and special. 

 The general mutations can be considered as parallel and taxonomic from the standpoint of 

 the systematist, and as progressive and retrogressive from the standpoint of the geneticist. 

 The parallel mutations appear in different species, as for example, the dwarfs which are pro- 

 duced every year by Oe. biennis and Oe. Lamarckiana, and the sulfurea form of Oe. biennis 

 and Oe. suaveolens. Parallelism is not limited to species of one genus but goes beyond these 

 limitations. For example, the cruciate form of sepals of Epilobium hirsutum cruciatum, and 

 very rare mutations of Oe. biennis cruciata. As an example of taxonomic mutation de Vries 

 cites the complete lack of petals in the mutant Oe. suaveolens. — The absence of petals is a 

 species character of Fuchsia macrantha and F. procumbens. Examples of progressive muta- 

 tions are those in which a double number of chromosomes occurs, — gigas forms. Among retro- 

 gressive mutations are Oe. nanella, Oe. brcvistylis and Oe. rubrinervis. The half-mutants are 

 those which are produced by the fusion of a recessive mutated gamete with a normal gamete, 

 as the mutant gigas. In this form we have annually 2 to 3 per cent mutants of the dwarf form. 

 The half-mutants, which can be isolated here, give 25 per cent plants of the gigas form, 50 

 per cent half-mutants and 25 per cent dwarfs. The first and third forms are constant. The 

 half-mutants lead us to the group of special mutations. The first example cited by author is 

 Oe. grandiflora. Two-thirds of the plants grown from seed are green and like the parent, 

 and one-third consists of yellow-green weak forms which die if left in the open. About one- 

 fourth of the seed are sterile. This phenomenon author explains in the following manner: 

 Oe. grandiflora is a half-mutant which segregates into 25 per cent ochracea forms, 50 per cent 

 half-mutant forms, and 25 per cent homozygous forms, the latter of which cannot be formed 

 because the factor for grandiflora is united with a lethal factor. Parallel with this is also the 

 appearance of Oe. Lamarckiana mut. rubrinervis, which segregates in Oe. deserens and Oe. rubri- 

 nervis. About half of the seeds of Oe. Lamarckiana are empty. This is explained by au'hor 

 in that Oe. Lamarckiana produces two kinds of gametes, the typical or laeta, and the velutina. 

 Each gamete has a lethal factor which is closely linked with the character factor. Heterozy- 

 gous combinations of these factors give good seeds which produce plants and homozygotic 

 combinations give the sterile seeds. If one of the two lethal factors becomes "vital" the 

 laeta or the velutina mutation appears. Finally he considers heterogamy, i.e., the phenomenon 

 in which the direct and the reciprocal crosses are not the same. He assumes that the species 

 which are crossed are half-mutations but that part of the pollen is lethal. — M. Demerec. 



350. De Wilde, P. A. Verwantschap en Erfelijkheid bij doofstomheid en retinitis pig- 

 mentosa. [Relationship and heredity in deaf-and-dumbness and retinitis pigmentosa.] Dis- 

 sertation, Amsterdam. 1919. — See also Bot. Absts. 4, Entry 520. 



